Purpose : To evaluate the therapeutic effect of gamma knife radiosurgery in cerebral vascular malformation using a radiologic imaging method such as MRI or angiography. Materials and Methods : We retrospectively reviewed MRIs, CT scans and angiograms of 29 cases of arteriovenous malformation and 15 of cavernous malformation before and after gamma knife radiosurgery. The patients underwent follow-up radiologic studies for between 6 and 35months after radiosurgery. No patient underwent other surgery of embolization. Radiological imaging analysis focused on changes in the volume of the nidus or central core. Other findings of edema, cystic change, hemorrhage, signal intensity, enhancement and distributional vascular markings were also studied. The volume of the AVM nidus was measured and assigned ot one of four groups : <1cc, 1-4cc,> 4-10cc and> 10cc. Results : In arteriovenous malformation cases, the volume of the nidus decreased by mean 60.2%; reduction was greatest(68.1%) in the 1-4ccgroup. Three cases showed complete loss of the nidus at 9, 12 and 25 months after radiosurgery, respectively. In nine cases, decreased caliber or loss of draining vein was noted. High signal intensity on T2-weighted MR images, suggesting either edema or demyelination, was observed in four cases. In cavernous angioma cases, core volume was reduced by 36.8%. Transient cerebral edema and presumed radiation necrosis were observed in two cases and one, respectively. Conclusion : Gamma knife surgery was effective in nearly all cases of arteriovenous malformation and some cases of cavernous malformation. More than two years follow-up involving radiologic imaging such as MRI is needed for the evaluation of therapeutic effect and diagnosis of complications.